The present invention relates to an analog to digital (A/D) converter for converting an analog signal to a digital signal.
A typical example of an A/D converter with high operation speed is the so-called successive approximation A/D converter which has been advantageously utilized in the form of a semiconductor integrated circuit. The A/D converter of this type operates as follows. First, an analog signal to be converted into a digital signal is sampled by a sampling signal and held. Then, the sampled analog signal is compared with an internal analog signal generated by a built-in digital to analog (D/A) converter. If the result of the comparison does not indicate the coincidence of the sampled analog signal and the internal analog signal, the value of the internal analog signal is changed so as to reduce the difference between the sampled analog signal and the internal analog signal until the internal analog signal becomes co-incident with the sampled analog signal. The digital value of the built-in D/A converter at the time when coincidence is detected is derived as the digital output corresponding to the sampled analog signal.
The successive approximation A/D converter is utilized especially in the field of audio signal processing.
The A/D conversion of an audio signal has been performed using a sampling signal at a frequency two or more times the frequency of input analog signal to be converted. Since noise of a frequency higher than that of the analog signal is generated through the sampling operation, an analog filter is provided to eliminate the noise prior to A/D conversion. However, it has been difficult to produce the analog filter in an integrated circuit device with high precision and with high reproducibility. In order to avoid the use of the analog filter, it is necessary to obtain a high-speed, simple-construction A/D converter as an integrated circuit, in which the analog signal together with noise is converted into a digital signal which is then applied to a digital filter, to discriminate the signal from the noise.